Nanoplasmonic cavities for photovoltaic applications

ABSTRACT

Nanoplasmonic cavities for photovoltaic applications include at least one transparent conductive substrate. A first plasmonic electrically conductive nanostructure layer is associated with the transparent conductive substrate. At least one photoabsorber layer is associated with the first plasmonic electrically conductive nanostructure layer. At least one electron transfer layer is associated with the photoabsorber layer. A second plasmonic electrically conductive nanostructure layer is associated with the electron transfer layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application, claiming the benefit of both parentnon-provisional application Ser. No. 13/658,573 filed on Oct. 23, 2012and parent provisional application No. 61/624,811 filed on Apr. 16,2012, whereby the entire disclosures of which are incorporated herebyreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein may be manufactured and used by or forthe government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

FIELD OF THE INVENTION

The invention generally relates to power devices, and more particularly,to improving the efficiency of portable photovoltaic (PV) power devicesand the sensitivity of visible and near-infrared photodetectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side perspective view of an excitonic photocell havingnanoplasmonic cavities, according to embodiments of the invention.

FIG. 2 is a side perspective view of an excitonic photocell depictinglight flow into and within nanoplasmonic cavities, according toembodiments of the invention.

FIG. 3 is a graphical comparison of light transmission through resonantnanolayers, according to embodiments of the invention.

FIG. 4 is a graphical comparison of the absorbance spectrum for apolythiophene film, according to embodiments of the invention.

FIG. 5 is an exploded view of stacked photocells having nanoplasmoniccavities, according to embodiments of the invention.

FIG. 6 is a plan view of a photovoltaic power source havingnanoplasmonic cavity arrays, according to embodiments of the invention.

It is to be understood that the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not to be viewed as being restrictive of the invention, as claimed.Further advantages of this invention will be apparent after a review ofthe following detailed description of the disclosed embodiments, whichare illustrated schematically in the accompanying drawings and in theappended claims.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention generally relates to power devices, and more particularly,to improving the efficiency of portable photovoltaic (PV) power devicesand the sensitivity of visible and near-infrared (IR) photodetectors.

Embodiments of the invention address a U.S. Navy need for lower cost,low mass, efficient photovoltaic power sources that are flexible anddurable. These PV power sources can be used for several applicationssuch as, for example, remote sensing stations and survival equipment.

Embodiments of the invention include a device fabrication process byelectrostatic sells-assembly of organic conjugated polyelectrolytes andatomic layer deposition of titanium dioxide. Embodiments utilize lightharvesting materials, such as electrolyte-substituted polythiophenes, toself-assemble conformal nanofilm coatings. The layer-by-layer deposition(ESD) technique enables precise (within a few nanometers) control overthe film thickness, the concentration gradient of each component of thefilm, and precise positioning of the maximum concentration of eachcomponent. The surface of titanium dioxide is a well-known efficientcharge-separation interface commonly used in dye-sensitized solar cells.The titanium dioxide further serves to transport electrons. In anexample of embodiments of the invention, titanium dioxide was depositedusing atomic layer deposition (ALD). ALD produces high quality wide areafilms with nanometer control of thickness. Using materials selected formaximum compatibility, multi-layer structures may be fabricated bymethods that have demonstrated capability for rapid scale-up for massproduction of large area devices.

Embodiments of the invention demonstrate atomic layer deposition oftitanium dioxide on electrostatic self-assembly of organic conjugatedpolyelectrolytes and plasmonic coupling of light to an embedded silverlayer. This embodies the construct and fabrication of an overlayer forPV devices which serves multiple functions, namely, antireflection,encapsulation, and most importantly, visible to near IR light focusingwith numerical aperture matched for efficient launching of plasmonicwaves into the metallodielectric nanolayers.

The nanostructures are constructed to enhance the absorbance of incidentphotons. The nanostructures are constructed to support propagating aswell as localized plasmonic modes. The substrate is alternately broughtinto contact with a solution (or dispersion) of positively-chargedparticles and a solution of negatively-charged particles. The resultingphotoabsorber film is conformal with thickness controllable to within+/− 1 nanometer (nm). In this example, the polyelectrolytes are bothfunctionalized polythiophenes. ALL) is a chemical vapor depositionmethod in which film thickness may be controlled to within +/− 0.2 nm.

Embodiments of the invention yield the following: (1) plasmonicenhancement of light absorbance, (2) nanostructures set up plasmonicinterference pattern thus trapping the light energy and leading to lightpropagation along plasmonic surfaces, and (3) polythiophene chains liein plane and absorb light more efficiently travelling parallel. Thisconstruction causes a fraction of the incident light to travel parallelto the chains.

Although embodiments of the invention are described in considerabledetail, including references to certain versions thereof, other versionsare possible such as, for example, orienting the layers in differentfashion. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of versions included herein.

In the accompanying drawings, like reference numbers indicate likeelements. FIG. 1 illustrates a side perspective view of an excitonicphotocell having nanoplasmonic cavities, according to embodiments of theinvention. Reference character 100 depicts an apparatus of embodimentsof the invention. The apparatus 100, an excitonic solar cell may also bereferred to with other descriptions including, but not limited to, aphotovoltaic power device without detracting from the merits orgenerality of embodiments of the invention. Likewise, variouscomponents/layers may be referred to with similar varying terminology.For example, some layers are referred to as first plasmonic electricallyconductive nanostructure layer in one embodiment and in a differentembodiment are referred to as a primary plasmonic electricallyconductive nanostructure layer without detracting from the merits orgenerality of embodiments of the invention.

Embodiments of the invention generally relate to an excitonic solar ell,including at least one transparent conductive substrate 102. A firstplasmonic electrically conductive nanostructure layer 104 is associatedwith the transparent conductive substrate 102. At least onephotoabsorber layer 106 is associated with the first plasmonicelectrically conductive nanostructure layer 104. At least one electrontransfer layer 108 is associated with the photoabsorber layer 106. Asecond plasmonic electrically conductive nanostructure layer 110 isassociated with the electron transfer layer 108. The electron transferlayer 108 is deposited by atomic layer deposition on the photoabsorberlayer 106.

Another embodiment of the invention generally relates to a process ofmaking an excitonic solar cell, including depositing a first plasmonicelectrically conductive nanostructure layer 104 on at least onetransparent conductive substrate 102. At least one photoabsorber layer106 is deposited on the first plasmonic electrically conductivenanostructure 104. At least one electron transfer layer 108 is depositedon the photoabsorber layer 106 by atomic layer deposition. A secondplasmonic electrically conductive nanostructure layer 110 is depositedon the electron transfer layer 108.

In yet another embodiment, the invention generally relates to anexcitonic solar cell made according to the process disclosed above. Theproduct by process in this example is a solar cell, more specifically,an excitonic solar cell. FIG. 2 illustrates a side perspective view ofan excitonic photocell depicting light flow into and withinnanoplasmonic cavities, according to embodiments of the invention. Thedepiction is represented as reference character 200. Excitonic solarcells have a charge separation interface. Charge separation occurs atthe interface of photoabsorber 106 and the electron transfer layer 108.As light 202 passes through the apparatus 200, a fraction of the photonsexcite electrons within the photoabsorber 106 to a higher energy level.The electron-hole pair (also referred to as an exciton) is short-lived.Excitons decay by a number of processes but predominately byrecombination. Plasmons are light energy flowing and are depicted asreference character 204. The plasmons 204 may transfer to a nearby metalsurface (transferred plasmons are depicted as reference character 208)through evanescent coupling (shown as reference character 206).

When the exciton is nearby a charge separation interface, the electronmay jump to the electron transfer layer 108 leaving a hole in thephotoabsorber 106. Typically, many excitons are produced and existsimultaneously. At the charge separation interface, this leads to higherconcentration of electrons on one side and high concentration of holeson the other. The electrons move away from the interface due to chargerepulsion and diffuse away from the region of high concentration.Likewise, at the other side of the interface, holes diffuse away.

In embodiments, the transparent conductive substrate 102 is typicallyconstructed by coating glass, plastic, or polyethylene terephthalate(PET) with transparent conductive materials such as, for example, indiumtin oxide, fluorine-doped tin oxide, carbon nanotubes, graphene, andaluminum-doped zinc oxide. In some embodiments, the first plasmonicelectrically conductive nanostructure layer 104 is deposited on thetransparent conductive substrate 102 by physical vapor deposition. Inother embodiments, the first plasmonic electrically conductivenanostructure layer 104 is deposited on the transparent conductivesubstrate 102 by electrodeposition.

In embodiments, the first and second plasmonic electrically conductivenanostructure layers 104 and 110 are selected from the group consistingof silver, gold, aluminum, copper, platinum, graphene, and carbonnanotubes. The photoabsorber layer 106 is deposited by electrostaticdeposition on the first plasmonic electrically conductive nanostructurelayer 104. Photoabsorber layer 106 materials may be conjugated polymers,dyes, semiconductors, such as, for example, silicon and germanium, andquantum dots.

The electron transfer layer 108 is at least one electron transfer film,such as, for example, titanium dioxide. In some embodiments, the secondplasmonic electrically conductive nanostructure layer 110 is depositedon the electron transfer layer 108 by physical vapor deposition, whilein other embodiments, the second plasmonic electrically conductivenanostructure layer 110 is deposited on the electron transfer layer 108by electrodeposition. A voltage meter 112 (FIG. 1) is shown to depictthat the apparatus 100 is a power device. Free space (air) is modeled tosurround the apparatus 100.

Significant modeling of embodiments of the invention suggests that filmand interface properties of thickness, purity, absorption, and relativeenergy level among others, are important parameters to control in thepursuit of more efficient excitonic solar cells. Embodiments of theinvention combine the film deposition techniques of electrodepositionand atomic layer deposition. Both of these techniques are well suitedfor large area, robust films. These techniques make efficient use ofmaterials leading to economical light weight solar cell production.Furthermore, the techniques are compatible with plasmonic nanostructuresthat will improve photoabsorbance efficiency.

Scattering matrix techniques using Maxwell's equations were used tosimulate embodiments of the invention. FIG. 3 is a graphical comparisonof light transmission through resonant nanolayers, according toembodiments of the invention, and is depicted as reference character300. FIG. 3 may also be thought of as a simulation of the plasmoniccavity. FIG. 4 is a graphical comparison of the absorbance spectrum fora polythiophene film, according to embodiments of the invention, and isdepicted as reference character 400.

In FIG. 3, the parameters used in the simulation are as follows:polythiophene equal to 25 nm in thickness, silver for the first andsecond plasmonic electrically conductive nanostructure layers 104 and110, each having a thickness of 28 nm, and titanium dioxide for theelectron transfer layer 108, having a thickness of 25 urn. A comparisonof FIGS. 3 and 4 indicate that a minima in transmission corresponds withan absorbance peak of polythiophene.

Another embodiment of the invention is illustrated in FIG. 5 and depictsan exploded view of stacked excitonic solar cells having nanoplasmoniccavities, according to embodiments of the invention. An apparatusaccording to this embodiment is depicted as reference character 500, andincludes at least one flexible transparent conductive substrate102A/B/C/D. At least one nanoplasmonic sandwich layer (the sandwichlayer includes reference characters 104A through 110A, 1040 through110B, 104C through 110C, and 104D through 110D is tuned to apredetermined spectral range. The flexible transparent conductivesubstrates 102A/B/C/D coupled with the respective nanoplasmonic sandwichlayers (reference characters 104A through 110A, 104B through 110B, 104Cthrough 110C, and 104D through 110D form stacked excitonic solar cells500.

The nanoplasmonic sandwich layers (reference characters 104A through110A, 104B through 110B, 104C through 110C, and 104D through 110D areassociated with the flexible transparent conductive substrate102A/B/C/D. At least one electrical lead 502 connects adjacent excitonicsolar cells such as, for example, reference character layers 102A/B,104A, 106A, 108A, and 110A with reference character layers 102A/B, 104B,106B, 108B, and 110B. Each of the flexible transparent conductivesubstrates 102A/B/C/D is electrically connected (the electricalconnection is depicted as reference character 504, which may be anyappropriate electrical connection such as, for example, an electricalbusbar) to at least one power storage device, depicted as referencecharacter 506, such as, for example, a capacitor.

The electrical busbar 504 completes an electrical circuit. Low energyelectrons flow in to fill holes vacated by the excited electrons. Theapparatus 500 may also be connected to power electrical devices such as,for example, electrical motors without detracting from the merits orgenerality of embodiments of the invention.

The flexible transparent conductive substrate 102A/B/C/D is configuredto fold and maintain electrical conductivity with layers 104A/B/C/D andas well as the electrical connection 504. The foldable nature of theflexible transparent conductive substrate 102A/B/C/D offers the abilityfor the apparatus 500 to be repeatable such that, for example, numerousexcitonic photocells (reference characters 104A through 110A, 104Bthrough 110B, 104C through 110C, and 104D through 110D) may be formedand stacked on top of one another. The flexible transparent conductivesubstrate 102A/B/C/D, at a molecular level, is at least partiallydisordered and amorphous at the processing temperature.

The nanoplasmonic sandwich layers (reference characters 104A through110A, 104B through 110B, 104C through 110C, and 104D through 110D arestructures that include at least one primary plasmonic electricallyconductive nanostructure layer 104A/B/C/D associated with the flexibletransparent conductive substrate 102A/B/C/D. At least one photoabsorberlayer 106A/B/C/D is deposited by electrostatic deposition on the primaryplasmonic electrically conductive nanostructure layer 104A. At least oneelectron transfer layer 108A/B/C/D is deposited by atomic layerdeposition on the photoabsorber layer 106A/B/C/D. At least one secondaryplasmonic electrically conductive nanostructure layer 110A/B/C/D isassociated with the electron transfer layer 108A/B/C/D.

Another embodiment of the invention generally relates to a process ofmaking stacked excitonic solar cell(s), including depositing at leastone primary plasmonic electrically conductive nanostructure layer104A/B/C/D on at least one flexible transparent conductive substrate102A/B and 102C/D. At least one photoabsorber layer 106A/B/C/D isdeposited on the primary plasmonic electrically conductive nanostructure104A/B/C/D. At least one electron transfer layer 108A/B/C/D is depositedon the photoabsorber layer 106A/B/C/D by atomic layer deposition. Atleast one secondary plasmonic electrically conductive nanostructurelayer 110A/B/C/D is deposited on the electron transfer layer 108A/B/C/D.

The deposition tasks described above are iterated through repeatedly toform stacked solar cells. The iteration includes the depositing of theprimary plasmonic electrically conductive nanostructure layer 104A/B/C/Don the flexible transparent conductive substrate 102A/B/C/D task; thedepositing of the photoabsorber layer 106A/B/C/D on the primaryplasmonic electrically conductive nanostructure 104A/B/C/D task; thedepositing of the electron transfer layer 108A/B/C/D on thephotoabsorber layer 106A/B/C/D by atomic layer deposition task; and thedepositing of the secondary plasmonic electrically conductivenanostructure layer 110A/B/C/D on the electron transfer layer 108A/B/C/Dtask. The iteration is performed a predetermined n number of times andis based on application specific conditions. The predetermined number nis selected from the range of whole numbers 2 through 10.

The completion of each iteration configures diametrically-opposeddistinct sets of stacked excitonic solar cell(s). The flexibletransparent conductive substrate 102A/B & 102C/D is folded thepredetermined number n minus 1 times. Each folding task is performedbetween each of the distinct sets of stacked excitonic solar cell(s), sothat each of the folding tasks juxtaposes adjacent distinct sets ofstacked excitonic solar cell(s) on top of each other. Folding techniquesare based on application-specific conditions. As such several foldingtechniques may be employed such as, for example, accordion-folding,without detracting from the merits or generality of embodiments of theinvention.

In yet another embodiment, the invention generally relates to anexcitonic solar cell made according to the process disclosed above. Theproduct by process in this example is a solar cell, more specifically, astacked excitonic solar cell.

In embodiments, nanoplasmonic cavities are the sum of the thicknesses oflayers 106 and 108. The predetermined spectral range for tuning is therange of about 350 to 700 nanometers. Tuning occurs by varying thethickness of the photoabsorber layers 106A/B/C/D and the electrontransfer layers 108A/B/C/D. In embodiments having stacked orientations,layers may be independently tuned for different spectral ranges. As anexample, layers 106A and 108A in FIG. 5 may be independently tuned to adifferent spectral range than layers 106D and 108D. Adjacent distinctsets of excitonic solar cell(s) are connected by at least one electricallead 502. The distinct sets of stacked excitonic solar cell(s) areelectrically connected to the power storage device 506. The electricallead 502 and electrical busbar 504 are conductive materials such as, forexample, aluminum, silver, and copper.

Another embodiment of the invention is illustrated in FIG. 6 and depictsa plan view of a photovoltaic power source having nanoplasmonic cavityarrays, according to embodiments of the invention. An apparatusaccording to this embodiment is depicted as reference character 600, andincludes an array of excitonic solar cells (reference character 100 inFIGS. 1 and 6). At least one electrical contact 602 is associated withthe array of excitonic solar cells 100. At least one electrical bus 604is associated with the electrical contacts 602. Individual excitonicsolar cells 100 in the apparatus 600 are as depicted in FIG. 1 anddescribed above.

Another embodiment of the invention generally relates to a process ofmaking a photovoltaic power source, including providing an array ofexcitonic solar cells 100. A first plasmonic electrically conductivenanostructure layer 104 is deposited on at least one transparentconductive substrate 102. At least one photoabsorber layer 106 isdeposited on the first plasmonic electrically conductive nanostructurelayer 104 by electrostatic deposition. At least one titanium dioxideelectron transfer layer 108 is deposited on the photoabsorber layer 106by atomic layer deposition. A second plasmonic electrically conductivenanostructure layer 110 is deposited on the electron transfer layer 108.The array of excitonic solar cells 100 are connected together with atleast one electrical contact 602. The electrical contact 602 terminatesat the electrical bus 604.

In some embodiments, the array of excitonic solar cells 100 areconnected in series by the electrical contact 602 and then to theelectrical bus 604. In other embodiments, the array of excitonic solarcells 100 are connected in parallel by the electrical contact 602 andthen to the electrical bus 604. The electrical contact 602 and theelectrical bus 604 are conductive materials such as, for example,aluminum, silver, and copper.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassedwithin the invention. The upper and lower limits of these smaller rangesmay independently be included or excluded in the range, and each rangewhere either, neither or both limits are included in the smaller rangesis also encompassed within the invention, subject to any specificallyexcluded limit in the stated range. Where the stated range includes oneor both of the limits, ranges excluding either or both of those includedlimits are also included in the invention.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

What is claimed is:
 1. A stacked photovoltaic device, comprising: afirst solar cell bonded to a first flexible transparent conductivesubstrate, wherein said first flexible transparent conductive substrateis electrically-connected to an electrical busbar; a second solar cellbonded to said first flexible transparent conductive substrate, whereinsaid first solar cell and said second solar cell are positioneddiametrically-opposed from each other, wherein said first solar cell andsaid second solar cell are electrically-connected to each other; a thirdsolar cell bonded to a second flexible transparent conductive substrate,said second flexible transparent conductive substrate iselectrically-connected to said electrical busbar; a fourth solar cellbonded to said second flexible transparent conductive substrate, whereinsaid third solar cell and said fourth solar cell are positioneddiametrically-opposed from each other, wherein said third solar cell andsaid fourth solar cell are electrically-connected to each other; whereineach of said solar cells, comprises: a first electrically-conductivenanostructure layer; at least one photoabsorber layer associated withsaid first electrically conductive nanostructure layer; at least oneelectron transfer layer associated with said at least one photoabsorberlayer; a second electrically-conductive nanostructure layer associatedwith said at least one electron transfer layer; and wherein saidelectrical busbar is electrically connected to at least one powerstorage device; wherein said first electrically-conductive nanostructurelayer and said second electrically-conductive nanostructure layer are 28nm thick of silver, said at least one photoabsorber layer is 25 nm thickof polythiophene, and said at least one electron transfer layer is 25 nmthick layer of titanium dioxide.
 2. The device according to claim 1,wherein said first flexible transparent conductive substrate and saidsecond flexible transparent conductive substrate are configured to foldand maintain electrical conductivity.
 3. The device according to claim1, wherein said association of said at least one photoabsorber layer andsaid at least one electron transfer layer have a predetermined spectralrange of about 350 to 700 nanometers.
 4. The device according to claim1, wherein said at least one electron transfer layer is titaniumdioxide.
 5. The device according to claim 1, wherein said at least onephotoabsorber layer is comprised of polythiophene.
 6. The deviceaccording to claim 1, wherein said electrical connection of said firstand second solar cells are electrical leads and wherein said electricalconnection of said third and fourth solar cells are electrical leads. 7.A stacked photovoltaic device, comprising: first and second solar cellsbonded to a first flexible transparent conductive substrate, said firstand second solar cells positioned diametrically-opposed from each other,wherein said first and second solar cells are electrically-connected toeach other; third and fourth solar cells bonded to a second flexibletransparent conductive substrate, said third and fourth solar cellspositioned diametrically-opposed from each other, wherein said third andfourth solar cells are electrically-connected to each other; whereineach of said solar cells, comprises: a first electrically-conductivenanostructure layer; at least one photoabsorber layer associated withsaid first electrically-conductive nanostructure layer; at least oneelectron transfer layer associated with said at least one photoabsorberlayer; a second electrically-conductive nanostructure layer associatedwith said at least one electron transfer layer; wherein said first andsecond flexible transparent conductive substrates areelectrically-connected to an electrical busbar, said electrical busbaris electrically-connected to at least one power storage device; whereinsaid first electrically-conductive nanostructure layer and said secondelectrically-conductive nanostructure layer are 28 nm thick of silver,said at least one photoabsorber layer is 25 nm thick of polythiophene,and said at least one electron transfer layer is 25 nm thick layer oftitanium dioxide.
 8. The device according to claim 7, wherein said firstflexible transparent conductive substrate and said second flexibletransparent conductive substrate are configured to fold and maintainelectrical conductivity.
 9. The device according to claim 7, whereinsaid association of said at least one photoabsorber layer and said atleast one electron transfer layer have a predetermined spectral range ofabout 350 to about 700 nanometers.
 10. The device according to claim 7,wherein said at least one electron transfer layer is titanium oxide. 11.The device according to claim 7, wherein said at least one photoabsorberlayer is comprised of polythiophene.
 12. The device according to claim7, wherein said electrical connection of said first and second solarcells are electrical leads and wherein said electrical connection ofsaid third and fourth solar cells are electrical leads.
 13. The deviceaccording to claim 2, wherein said folding is accordion folding.
 14. Thedevice according to claim 8, wherein said folding is accordion folding.